Polyols are used in the preparation of polyurethanes which are widely used in the manufacture of automotive interior parts, furniture, elastomers, coatings, and others. A flexible polyurethane is prepared using a polyol having a relatively high molecular weight of 3,000 to 6,000, and a rigid polyurethane, using a polyol having a relatively low molecular weight of 150 to 1,000. Also, for the manufacture of a polyurethane slab, a polyol produced by random copolymerization using propylene oxide (PO) and ethylene oxide (EO) is commonly used.
The preparation of a polyol is generally conducted by the polymerization of one or more hydrocarbon epoxides in the presence of a polymerizing initiator and an alkaline catalyst such as KOH. During the epoxide polymerization, undesirable unsaturated polyols are produced through side reactions. Such a unsaturated polyol containing a carbon-carbon double bond and one hydroxyl group tends not to form a 3-dimensional net structure when polymerized with isocyanate, leading to poor physical properties, particularly unsatisfactory elasticity of the resulting polyurethane product. Also, the KOH catalyst used in the preparation of a polyol must be separated from the polyol product by filtering under a reduced pressure. Thus, the conventional procedure is hampered by poor productivity.
Accordingly, in order to reduce the amount of the unsaturated polyols generated during the conventional polyol synthesis, a double metal cyanide (DMC) catalyst has been used in place of the KOH catalyst. The DMC catalyst makes it possible to minimize the content of the unsaturated polyol in the polyol product (the degree of unsaturation) to a level of about 0.005 meq/g. Such DMC catalysts are used in the preparation of a variety of polymers, including polyether, polyester and polyetherester.
A DMC catalyst is conventionally prepared from a mixture of a metal salt, a metal cyanide salt and a complexing agent, and it can be represented by Ma[M′(CN)6]b LcL′d, wherein M and M′ are metal elements; L and L′ are complexing agents; and a, b, c and d are integers, the sum of a, b, c and d being equal to the sum of the charges of M and M′.
The complexing agent contained in the DMC is to enhance the activity of the catalyst, and a preferred complexing agent is ethylene glycol, dimethyl ether, alcohol, aldehyde, ketone, ether, ester, amide, urea or nitrile. For example, U.S. Pat. Nos. 4,477,589, 3,821,505 and 5,158,922 disclose ethylene glycol or dimethyl ether as a complexing agent; U.S. Pat. No. 5,158,922, alcohols, aldehydes, ketones, ethers, esters, amides, urea and nitriles; U.S. Pat. No. 5,780,584, tert-butyl alcohol; and U.S. Pat. Nos. 5,482,908 and 5,789,626, a polyether.
A polyol produced using a highly active DMC catalyst has a very low content of unsaturated polyols of about 0.005 meq/g, but such a polyol has properties greatly different with those of the polyol having an unsaturation degree of 0.03 meq/g used widely in the art, which necessitates the significant modification of the conventional polyurethane manufacturing process. Also, the highly active DMC catalyst is excessively reactive, which makes it difficult to control the reaction temperature because it induces a sudden exothermic reaction.